Trough waveguide to coaxial-line transformation



July 16, 1957 E G. FUBINI ETAL 2,799,830

TROUGH WAVEGUIDE To CQAXIALVLINE TRANSFORMATION Filed Aug. 30, 1955 ,4 lo Il as l2 36 26 FIG 6 INVENTORS EUGENE G. FUBlNl BYHENRY S. KEEN- ATTORNEYS nited tates Patent TRQUGH 'WAVEGUEDE TO CAXIAL-LNE TRANSFRMATIN Eugene G. Fubini, Glen Head, and Henry S. Keen, Uniondale, N. Y., assignors to Airborne Instruments Laboratory, Inc., Mineoa, N. Y., a corporation of Delaware Application August 30, 1955, Seriai No. 531,324

9 Claims. (Cl. S33-24) This invention relates to trough waveguide to coaxialline transformations.

In an application of Eugene G. Fubini for Microwave Circuits, Serial No. 531,323, led concurrently herewith, a so-called trough transmission line is desciibed which has sidewalls and a central member (which may be of finlike shape) positioned therebetween with the conductive sides of the central member conductively connected to the sidewalls at the bottoms thereof, but the top wall may be left completely open. This structure facilitates the insertion of probes for measuring or other purposes. Also, the trough functions generally on waveguide principles, but has an advantage over conventional rectangular waveguide in that the bandwidth for the dominant mode of operation may be made considerably greater before higher modes of operation are possible.

The aforesaid application describes a rectangular waveguide to trough waveguide transformation. In many applications it is desirable to be able to transfer power from a coaxial transmission line to a trough waveguide, or vice versa. In accordance with the present invention, a trough waveguide to coaxial-line transformation is provided which enables power to be fed in either direction with low insertion loss, good impedance match, and low standing wave ratio.

The transformation of the present invention may be used wherever desired. For example, it may be used to connect extended sections of coaxial-line and trough waveguide. Or, advantage may be taken of the high pass characteristic of the trough waveguide to eliminate undesired low frequencies in a coaxial transmission line. Such an arrangement is shown in one of the specific embodiments herein.

The invention will be described in connection with specic embodiments thereof and advantageous features thereof will in part be pointed out and in part be obvious to those skilled in the art.

In the drawings:

Figs. l, 2 and 3 are corresponding figures of the aforesaid application, and are shown here for convenient understanding of the present invention;

Fig. 4 is a perspective view, partially broken away, of trough waveguide to coaxial-line transformations in accordance with the present invention, the transformations at the two ends differing in detail to illustrate alternative arrangements;

Fig. 5 is a sectional detail of one end of Fig. 4; and

Fig. 6 illustrates a modified coupling arrangement.

Referring now to Fig. l, a portion of a trough transmission line or waveguide is shown comprising conductive sidewalls 10 extending transversely from a bottom wall 1l, and a central conductive member 12 positioned between the sidewalls and likewise extending transversely from the bottom wall 11. The height of the sidewalls 10 is substantially greater than the height of central member 12. rIhe central member 12 is lin-like in appearance, as specifically shown in Fig. l, but may be made considerably wider if desired. The several walls and bottoms extend longitudinally of the line, as indicated by the arrow 13, and electromagnetic energy propagates down the line in the direction indicated by the arrow.

Preferably the sidewalls 10 and central member i2 are fiat and parallel, as shown, with the central member positioned midway between the sidewalls. Also, preferably the bottom is dat, as shown. However, if desired, considerable departure from the configuration illustrated is possible. For example, the sidewalls might be bowed and also the bottom given some curvature. It is possible also to depart somewhat from the iiat configuration of the central member 12. Furthermore, the sidewalls can be at an angle, rather than parallel, so as to reduce the width of the bottom surfaces 11', and it is possible to reduce the width of the bottom surfaces to substantially zero. In the latter case, the conductive sides of central member 12 join the sidewalls at the bottoms thereof respectively. These modifications are described in the aforesaid application and it will be understood that the present invention is applicable thereto.

Generally, it is desirable to make the structure symmetrical about a plane passing through the central inember 12, so that one side of the line is substantially a mirror image of the other, in order to avoid spurious modes.

The entire structure shown in Fig. l may be made of a suitable metal of high conductivity. However, since high frequency current flow is confined essentially to the inner surfaces of the sidewalls, designated lil', the upper surfaces 11 of the bottom, and the sides i2' of the central member, if desired only these surfaces need be of conductive material. lf the entire unit is made of relatively inexpensive metal, the designated surfaces can be silverplated or otherwise given a highly conductive surface coat ing so as to minimize losses.

Further modifications are possible which may be desirable for particular applications. As will be explained hereinafter, substantially no component of current exists perpendicular to the free edge l2 of the central member. Consequently it is possible to employ a sheet of insulating material covered on both sides with a conductive coating or foil, etc. as the central member 12. Eurther, although not preferred, it is possible to use two spaced sheets of conductive material as the central member 12, with a narrow opening therebetween to allow insertion of a narrow probe or other coupling means. ln such case the two sheets will be joined to respective bottom surfaces, and the overall arrangement can be obtained by placing two U-shaped troughs beside each other, with the inner vertical surfaces shorter than the outer.

Fig. 2 shows the approximate configuration of the electric and magnetic fields for the dominant mode of operation, it being understood that the dominant mode is that mode of operation which has the lowest cut-@ frequency, and is the mode commonly employed in practice. In Fig. 2 the full lines 1d represent the eld lines or lines of flux of the electric vector, and the dash lines 15 represent the field lines or lines of linx of the magnetic vector. It will be noted that the field lines of the electric vector extend from the conductive sides i2 of the central member to the inner surfaces of the respective sidewalls 10. The spacing between adjacent field lines ivi is an indication of the relative intensity of the electric eld. At the bottom of the trough, near surfaces 11', the intensity of the electric field is small. The field becomes progressively more intense in the upwards direction along the central member and is a maximum at the free edge of the central member. On the other hand, transverse currents on the sides of the central member 12 vary from a minimum at. the free edge to a maximum at its base. The eld lines of the electric vector are generally parallel vrespective sidewalls.

to the bottom surfaces in the regions where the central member 12 lies directly opposite the sidewalls. In the regions above the free edge of the central member, the iield lines extend upwards along curved paths tothe It Vwill be recognized that-the field lines of the electric vector are everywhere substantially transverse to the `direction of propagation down the line.

In Fig. 3 the iield lines of the electric vector are again shown by full lines 14 extending between the central member 12 and the sidewalls 10, and the intensity of the ieldris indicated by the spacing of the lines. Since Fig. 3 is a longitudinal section, the field lines of the magnetic vector are seen end-on, solid dots 15 representing the t A 2,799,830 't magnetic vector directed toward the observer and open the top of the structure may be capped, if desired, to` V- keep the line free of dust and moisture. Y Generally the spacing between thek sidewalls is less than a half wavelength at the highest operating frequency, and the height .of the sidewalls exceeds that of central member 12 by at least the spacing of the sidewalls, to reduce transverse radiation. If caps of low dielectric constant approaching 'that of air are employed, the spacing and height of the sidewalls may be substantially tne same as that employed for physically open-top line. For materials of higher dielectric constant, and for conductive materials, it is desirable to further increase the height of the sidewalls for a given spacing until the field in the region of the cap is negligibly small. If desired, of course, such a conductive cap may be made integral Vwith the sidewalls rather than removable. Y Y v Y Referring now to Fig. 4, a section of trough transmission line is shown comprising sidewalls 10, bottom wall 11 and central member 12. At opposite ends provision is made to couple to coaxial lines through couplings Y 21 and 22. As shown, the two end sections differ slightly in detail to illustrate possible variations,'but they may be made the same if desired.

At the right-hand end of Fig. 4, a conductive end wall 23 is provided which is conductively connected to the ends of sidewalls and to the bottom 11. The left-hand end is similar except that a gap is left between the' bottom of end wall 24, designated 24', and the adjacent end of bottom 11.

Fig. 5 is a sectional view illustrating the transformation at the right end of Fig. 4. As shown, end wall 23 has an aperture 25 through which the central conductor 26 of a coaxial line section extends. A threaded coupling member 21 is affixed to the end Wallin any desired manner and contains an insulating bead 28 which holds conductor 26 in position. A section of conventional coaxial Vline 27 may be attached lto coupling member 21 Yby means of a threaded connector 29. The central con- Y ductor 31 of the coaxial line is provided with a clip 32 for convenient connection to conductor 26, or is otherwise arranged so that Vconductor 31 makescontact withv conductor 26. In eiect, coupling member 21V and the central conductor 26 therein forms a short section of coaxial line which is adapted for convenient attachment Vof a longer section of coaxial line. Other mechanical arrangements may be provided to lead a coaxial liner y The central conductor 26 is conductively connected to thencentral member 12 at a point 33which will provide a desired impedance match. The impedance of the central member approaches zero where it joins the bottom 11, and increases approximately sinusoidally squared to. a maximum at the top or free edge of the central member. Thus the point 33 may be selected so that the impedance of the central member at that point substantially matches the impedance of the coaxial line, or bears a desired relationship thereto.

In the specific embodiment of Fig. 5, the height of the central member 12 is gradually increased near the end wall, as indicated at 12". As explained in the aforesaid application, the higher the central member the lower the cutoff frequency for a given sidewall spacing. I-Ience increasing the height of the central member near the end wall makes the cutoff frequency of the trough trans-Y of connection to the coaxial line, the impedance at'the transition in the desired operating region is not excessively high and permits convenient matching.

From the previous discussion it will 'be understood that the Voltage along the sides ofthe central member 12 approaches zero at the base thereof and Vincreases to a lmaximum at the free edge. Accordingly, the end of the central member adjacent the end wall 23 is spaced there- Yfrom to prevent large deformation of the electric ield configuration. As actually shown, the bottom of the end of central member 12, designated 34, is slightly spaced from the end wall 23 and the end 35 tapers away from the end wall. The spacing of point 34 from the end wall may not be necessary in some instances, so long as the taper is suicient. On the other hand, in'stead of tapering the end 35, it could be made vertical and spaced from Ythe end wall. Y

Instead of conductively joining end wall 23 with the sidewalls and bottom of the trough section, it is possible to use other forms of R.F. coupling.

Fig. 6 shows an alternative method of coupling the central conductor 26 of the coaxial line to the central member of the trough section. Instead of a direct conductive coupling as shown in Fig. 5, capacitive coupling is employed. To this end, a small metallic section 36 may be secured to the end of conductor 26 `and placed in proximity to the edge 35 of the central member. The spacing and vertical location of member 36 may be selected to provide the desired degree of coupling' 'and impedance match. y

In both Figs. 5 and 6, the central conductor 26 of the coaxial line excites a current from a point on the central member 12 to the base thereof, which is normally at ground potential, thus giving rise to the lields illustrated in Figs. 2 and 3 for the dominant mode of operation.

' The conguration of the electric eld in the coaxial line closely resembles the upper portion of the electric eld near the free edge of the central member, as shown in Fig. 2, thus avoiding excessive discontinuities. y

In one specific embodiment which has been operated with success, a construction similar to that shown in Fig. 4 was employed. The sidewalls 10 were 3 inches high and spaced a half inch apart, and the central member 12 was 0.89 inch high. At the ends the central member increased to 1.27 inches in height in a distance of 2.12 inches. The impedance of a 50-ohm coaxial line was vsubstantially matched by connecting the center con-V f ductor thereof to the end of the central member about 0.79 inch from the base thereof. The upper end of Ythe end 35 was Yapproximately r3/32 inch fromethe end wall. A good impedance match was obtained over the desired range of 300D-6000 mc.

This example is merely illustrative, and is given vas an aid to the ready practice of the invention.

As mentioned in the discussion of Figs. 1 3, if the sidewalls of the trough waveguide are sufficiently higher than the central member, a 4conductive or insulating cap can be employed, and in such event the top of the embodiment shown in Fig. 4 may be capped.

ln the discussion of Figs. l-3, a number of modifications of the trough waveguide have been mentioned. it is believed that it will be obvious to a person skilled in the art how to make any necessary alterations in the transformations of Figs. 4-6 to suit the modifications in the trough waveguide itself. Generally it is advantageous to keep the transformation symmetrical about a plane through the central member of the trough waveguide. Further modifications are possible within the scope of the invention, and may be made as suits the designer or meets the need-s of a particular application.

In the specification and claims use has been made of terms such as bottom, top, sidewalls, height, etc. in order to define the relationships in convenient langauge which can be readily understood. The employment of such terms, however, is not intended to mean that the transformation must be used in practice with the top up, the bottom down, etc., as specifically illustrated, since the trough may be inverted, or laid on its side, etc., as meets the requirements of a particular application.

We claim:

l. A trough waveguide to coaxial-line transformation which comprises a trough waveguide section having a pair of spaced conductive sidewalls and a central member having conductive sides positioned between the sidewalls with said conductive Isides conductively connected to said sidewalls at the bottoms thereof respectively, the height of said sidewalls being substantially greater than the height of the central member, a conductive end wall coupled to said sidewalls, at least the upper portion of the end of 'said central member adjacent said end wall being spaced therefrom, an aperture in said end wall and a central conductor in said aperture arranged to form a coaxial-line section, said central conductor being coupled to said central member at a point removed from the bottom thereof,

2. A trough waveguide to coaxial-line transformation which comprises a trough waveguide section having a pair of spaced conductive sidewalls and a central member having conductive sides positioned between the sidewalls with said conductive sides conductively connected to said sidewalls at the bottoms thereof respectively, the height of said sidewalls being substantially greater than the height of the central member, a conductive end wall coupled to said sidewalls, the end of said central member adjacent said end wall being of greater height than portions thereof more remote from the end wall and at least the upper portion of said end being spaced from the end wall, an aperture in said end wall and a central conductor in said aperture arranged to form a coaxial-line section, said central conductor being coupled to said central member at a point removed from the bottom thereof.

3. A trough waveguide to coaxial-line transformation which `comprises a trough waveguide section having `a pair of spaced sidewalls, a central member having conductive sides positioned between the sidewalls and a bottom connecting the sidewalls with the conductive sides of the central member, said sidewalls and central member extending transversely from said 'bottom and the height of the sidewalls being substantially greater than the height of the centr-al member, a conductive end wall coupled to said sidewalls, the end of said `central member adjacent said end wall `being spaced therefrom, an taperture in said end wall and a central conductor in said aperture arranged `to form a coaxial-line section, said central conductor being connected to the end of said central mem-ber at a point removed from the bottom thereof.

4. A trough waveguide to coaxial-line transformation which comprises a trough waveguide section having a pair `of spaced sidewalls, a ycentral member having conductive sides positioned between the sidewalls and a bottom connecting the sidewalls with the conductive sides of the central member, said sidewalls :and central member extending transversely from said bottom yand the height of the sidewalls being substantially greater than the height of the central member, a conductive end wall coupled to said sidewalls, the end of said central member adjacent :said end wall gradually increasing to a greater height than portions thereof more remote from `the end wall and said end being spaced from the end wall, an aperture -in said end wall and a central conductor in said aperture arranged to form -a `coaxial-line section, said central conductor being connected to the end of said `central member lat a point removed from the bottom thereof.

5. A trough to coaxial-line transformation which comprises a trough waveguide section having a substantially at `conductive bottom and substantially fiat conductive sidewalls perpendicular thereto, and a substantially fiat central conductive member extending perpendicularly from said bottom midway between said sidewalls, said bottom, sidewalls and central member extending longitud-inally of said section and the height of the sidewalls -being substantially greater than the height of the central member, :a transverse conductive end wall connected -to said sidewalls, the end of said central member adjacent said end wall lbeing spaced therefrom, an aperture in said end wall and a central conductor spaced therein lto form a coaxial-line section in substantially longitudinal alignment with said central member, said central conductor being coupled to the end of said central member at a point removed from the bottom thereof.

6. A trough to coaxial-line transformation `which comprises a trough waveguide section having a substantially fiat conductive bottom and substantially liat yconductive sidewalls perpendicular thereto, and a substantially flat central conductive mem-ber extending tperpendicularly from said bottom midway between said sidewalls, said bottom, sidewalls and central -member extending longitudinally Iof said section and the height of the sidewalls being substantially greater than the height of the central member, a transverse conductive end wall coupled to said sidewalls and bottom, at least the ycoupling to said sidewalls being conductive, the end of said central member 'adjacent said end wall gradually increasing to a greater height than portions thereof more remote from `the end Wall and said end being spa-ced from the end wall, an aperture in said end wall and a central conduct-or spaced therein to form a coaxial-line section in substantially longitudinal alignment with said 'central member, said central conductor being connected to the end lof said central member at a point removed `from the bottom thereof.

7. A trough to Vcoaxial-line transformation which cornprises `a trough waveguide -section having a substantially at conductive bottom and substantially flat lconductive sidewalls perpendicular thereto, :and a substantially flat central conductive member extending perpendicularly from said bottom midway between said sidewalls, said bottom, sidewalls and lcentral member extending longitudinally of said section and the height of the sidewalls being substantially greater than the height Iof the central member, a transverse conductive end wall connected to said sidewalls, the end of said central member adjacent said end wall being spaced therefrom, an aperture in said end Wall :and a central conductor spaced therein to form a `coaxial-line Vsection in substantially longitudinal alignment with said `central member, said central conductor being 'coupled to `the end of said central member at a region removed from the bottom thereof.

8. A .trough to coaxial-line Itransformation which com- Y lductive: sidewalls and.V a 'central member havingycondllcr tive'sidespositioned? jbetween the sidewalls with' saidconductive sides conductively connected to said sidewalls Vat the bottoms thereoftrespcctively, the 'height of said sidewalls being substantially greater than the height lof the; central member, a conductive Vend wall coupled; to; said; sidewalls, at least the upper portion lof the end of said central lmember ladjacent said end wall being spaced therefrom, an aperture in said conductive end Wall andf a.. central `conductor in said yaperture arranged to yform a.

coaxialdine section, said central conductor being Vcoupled to said ycentral ymember 'at a region removedY from the bottom thereof.

9. A trough to coaxial-line transform-ation which conm` prises.v a trough waveguide sectiou'having a pair of con- `ductiveV sidewalls and a central member having conduc-V tive sides positioned between lthe sidewalls with said' conductivefsides conductively connected' t-ojsaid sidewalls at the bottoms thereof respectively, the height, of saidA sidewalls beingA substantially greater than the height of theV central member, ay transverse conductive endV wall- `coupled to said sidewalls, at least the upper portionof 'the end 'of said central member `adjacent -said end walls being spaced therefrom, an aperture in said `conductive end wallV and a'central conductor in said aperture arranged to -form a coaxial-line section, said central conductor being in substantially longitudinal alignment with said fcentralmember and being coupled to said central member at a. region removed from the bottom thereof.

No references cited. 

